Can Marijuana Kill You?

A recent headline reads: “Can Marijuana Kill You? German Scientists Say Yes.” The article focuses on a study of two (count ’em, two!) young men who died while they had detectable levels of THC in their blood. I take a lot of pleasure in this kind of melodrama.

By Mitch Earleywine

If prohibitionists are stooping this low, we must really be frightening them. (It’s not completely pharmacologically ridiculous. Marijuana does increase heart rate. In fact, it can jack up heart rate almost as much as an espresso or energy drink. Maybe if you already had a weak heart and a coffee and a bong hit, well, something might happen.)

But I want to point out that we should actually expect literally thousands of reports like this. We should hear about lots of people who have heart attacks on the same day that they commune with the plant. It’s not because cannabis causes heart attacks. It’s simple chance.

I hate for my first blog as Chair of The Executive Board to be this nerdy, but I’ve been teaching statistics for more than 20 years. If that doesn’t make me a nerd, I’m not sure what would. But given how many people use cannabis daily and how many heart attacks occur in the United States, it’s actually a miracle that we haven’t heard about this kind of thing before. We also should expect to hear it a lot more often.

According to the National Survey on Drug Use and Health, roughly 7,600,000 Americans (over age 12) used marijuana daily or near daily in 2012. In addition, the Center for Disease Control suggests that about 715,000 of us have heart attacks in a year. (Let’s assume those under age 12 are probably not grabbing their chests with a myocardial infarction too often.) In addition, let’s guess that the United States has about 280 million people over age 12. It’s hard to know the exact number, but that’s probably in the ballpark.

With this in mind, we can predict how many people should have a heart attack the same day that they used cannabis simply by chance. That is, even if these two things had nothing to do with each other, we should expect some folks to have a heart attack the same day that they used cannabis just by accident.

Okay. It’s going to get nerdy here, but this is comparable to asking simpler questions. If I had a dime and a nickel, I might want to know what the chances are that I’d flip heads on both. I flip heads 1 out of 2 times on average for the dime, for a probability of .5. Then I flip heads on the nickel 1 out of 2 times on average, also for a probability of .5. So the chances of flipping heads on both is .5 * .5 for .25. So we’d expect to get heads on both coins about 1Ž4 of the time. If I flipped both coins 100 times, I’d get around 25 pairs of heads. Note that there’s nothing causal here. The nickel doesn’t know what the dime did. It doesn’t want to be like the dime. It’s not that the dime caused the nickel to flip heads.

So it’s the same deal for the cannabis-related heart attacks. If 7.6 million people use cannabis daily out of 280 million relevant Americans, that’s a probability of .0271. And if 715 thousand of 280 million have heart attacks, that’s a probability of .0026. Multiply these the same way we did with the probabilities for flipping heads (.0271 * .0026 = .00007). Now .00007 is a dinky number. If there were only 100 people in the country, we wouldn’t expect any of them (well, .007) to have a heart attack and smoke cannabis on the same day. But we’re talking about 280 million people here. So we’d expect .00007 * 280,000,000, = 19,600. That’s over 19,000 heart attacks.

So the question isn’t, “How did these two guys die of a heart attack with THC in their blood?” It should be, “Where are the other 19,598 guys who should have had heart attacks with THC in their blood?” In fact, the absence of this many cannabis-related myocardial infarctions inspired my wife to ask, “Does cannabis protect the heart?”